Welcome back. Glad to hear that last year's trip was a success.

The places you mention could all be visited in the time you have available. Allow a couple of days for DC. Even just going to the many Smithsonian sites will take a while, besides all the monuments, etc. Unless you already have a car, you could consider taking the train to DC and picking up a car when you leave there. Public transport is very good in the city, and parking is expensive. Besides, you can spend a lot of time looking for somewhere to park.

Do you have passports to go over into Canada? Niagara Falls is a great place to visit, especially early in the season, and the Canadian side is really the best. There's also a great IMAX movie which is shown on the Canadian side, which gives a lot of information on the falls. It is also the side where the falls are illuminated at night. The Maid of the Mist tour operates from both sides, but the walk behind the falls (if it is operating in early May) goes from the Canadian side.

Have you thought of traversing back to NYC via Boston. A city with much history, especially of early European settlement.

Lifey